Purple sea plume Gorgonian
(Phylum Cnidaria, Class Anthozoa, Order Gorgonacae. See Overview of Cnidarian Diversity)
About Gorgonians:
Gorgonians are members of the octocoral group (Subphylum Alcyonaria), which along with the soft corals and some others have polyps with tentacles in multiples of eight. In contrast, stony coral polyps have tentacles in multiples of 6.
Gorgonians are distinguished from most of the other cnidarians by their stiff but flexible internal skeletons. Though these skeletons have the appearance and texture of wood, they are in fact made of a protein called gorgonin.
Though there are some gorgonians that form non-branching colonies that encrust rocks, most species develop into tall, branching colonies. Examples include the sea fans, sea whips, sea plumes, sea rods, etc.
Though some gorgonians (e.g. sea fans) are very difficult to keep in aquaria because it is difficult to provide them with adequate amounts of the foods they need, all of our gorgonians have zooxanthellae (symbiotic algae in their tissues) that provide for much of their energy needs. Such photosynthetic gorgonians are quite hardy and grow well in a healthy reef aquarium.